The presently disclosed embodiment relates to a process of drape forming composite materials and to a material suitable for this process.
It applies to the use of thermoplastic composite materials in drape forming to produce large structural parts of complex shape.
Within the context of the drape forming of composite materials, it is necessary to enable the coupling of a first layer of composite material to the tool in order to enable the drape forming of the part.
The solutions that enable the drape forming must also be compatible with an automated drape forming using laser heating of the composite.
Thermoplastic materials do not have a tacky nature at ambient temperature. This state prevents the coupling thereof to a tool during the use thereof. Although simple means, for example adhesive tape, exist for small flat formers, the problem has not been solved in order to allow the coupling of sheets laid up on complex formers. If the first ply cannot be coupled to the tool, this will prevent the production of the part.
In order to solve this problem, it is advisable to develop a technical solution that will enable the coupling of a first ply to a complex tool during the laying up thereof. This solution must also enable the part to be removed from the mold at the end of the drape forming or consolidation. Finally, this solution must make it possible to retain the drape-forming precision of the machine, specifically excessive deformation of the first ply could disturb the lay-up and lead to gaps and/or overlaps of the laid up sheets which would adversely affect the soundness of the final material and also the mechanical performances thereof.
The problems that must be solved by the solutions to the problem of drape forming large parts, in particular of complex or non-developable shape, using a tool that represents a former of the part, are the following:
deformation/conformation on tool;
compatibility with the matrix of the composite;
coupling of the composite to the solution;
stability during the drape forming;
ability to be removed from the mold after consolidation.
Among the solutions identified in the past are:                the production of a hoop,        applying a vacuum,        the use of a polymer film and in particular of a PEEK film.        
A hoop consists in carrying out winding around the tool on which the drape forming is desired. The fact of having a continuity of fibers around the tool combined with peripheral mechanical clamping (adhesive tape for example) makes it possible to hold the material on the drape forming zone. This first layer enables the whole of the part to be laid up, but generates many losses on the rear face of the tool since the material of the winding on this face will be unused thereby. This solution nevertheless remains a reference solution for rotational parts such as cylinders.
The technique of applying a vacuum consists in holding the material on the drape-forming zone by means of a vacuum created by suction. For this, the drape forming is carried out on a perforated surface connected to a suction circuit. When the drape forming begins, the suction circuit is activated and the material is held on the working zone. However, the material, for example, carbon fibers, penetrates into the suction orifices which deforms it and leads to a degradation of the properties of the final material. This solution furthermore has a cost premium for the process since it requires the production of a complex tool with integrated vacuum circuit.
Finally, the use of polymer films is not very well suited to the drape forming of complex shapes since the latter will lead to creases of the film that will generate variations in accuracy during the drape forming and local overthicknesses in the material. The use of a film requires a complex assembly in order to ensure it is held in position.
It is also possible to use a multi-perforated tool with an integrated vacuum circuit that is expensive or else to use the polymer film as a vacuum cover which requires the use and installation of drainage fabrics and of sealing mastic.
The latter option combines the drawback of requiring several hours of manual labor, which is detrimental to the operating efficiency of an automated drape forming machine, adding increased sensitivity in the case of a problem during the production of the first ply. This sensitivity is illustrated when it is necessary to remove the laid up rovings which are welded to the surface of the film, the latter tearing or having to be cut in order to carry out this operation. The system thus loses its leak tightness and can no longer meet the dimensional stability constraints nor those of the holding of the composite at the surface of the tool. It is thus necessary to replace the film or effect a repair in order to be able to carry out the drape forming, with, as a result, new manual operations which will be all the more costly in terms of time and materials when this occurs on a large-sized part.